Is it possible to visualize the ray paths in the 3D viewport?
I've made a tutorial for you to explain all the steps.

The essential part is that you can't simply mirror a camera along an axis you need to parent the camera to an object/empty first and then operate on the parent to mirror it.
The video show other tricks like "clear parent inverse" to align a plane on the same camera plane. 
Also "Copy Attribute" addon was used to copy the rotation (but you can do it manually)

Hope this answer your question if it does upvote :)